The invention relates to a draw-through gripper for the insertion of an auxiliary weft thread into the seam-weaving shed in a seam-weaving machine. The gripper has a movable gripping arm with a gripping collet for grasping the auxiliary weft thread.
Industrial plastics woven fabrics for uses in which a very regular surface structure of the woven fabric is required, especially flat-woven plastics paper-forming fabrics, are made endless by a woven seam. To produce a woven seam, warp threads are exposed to a length of e.g. 15 cm at the woven fabric ends which are to be joined to each other, the weft threads in this zone being removed. The so-called woven seam, in which the original weave binding is exactly reproduced, is then formed from these warp thread fringes and the weft threads removed from the woven fabric end. An auxiliary weaving shed or seam-weaving shed is spread out from the removed weft threads, in which the removed weft threads function as auxiliary warp threads. The warp thread fringes are inserted into this seam-weaving shed as auxiliary weft threads alternately from the two woven fabric ends.
Of the plurality of warp thread fringes projecting from every woven fabric end, one warp thread fringe is singled out and held fast by means of a separator (DE-U-87 13 074, EP-A-0 301 174 and DU-U-90 02 278). A handover gripper transports this warp thread fringe to a draw-through gripper which then inserts it as an auxiliary weft thread into the seam-weaving shed, so that the auxiliary weft thread initially lies taut in the seam-weaving shed. The draw-through gripper is of the design mentioned initially and is known e.g. from DE-U-81 22 449, EP-A-0 043 441 and EP-A-0 236 601. The draw-through gripper is so designed in modern seam-weaving machines that it simultaneously checks for the presence of the warp thread fringe, so that woven-seam faults which would result from a missing warp thread fringe are avoided. If a warp thread fringe is missing, the seam-weaving machine is stopped so that the fault can be eliminated at once and substantial reworking thereby avoided. To monitor the presence of a warp thread fringe, the griping collets of the draw-through grippers are usually designed as electrical switches with two switch contact surfaces which touch one another when there is not warp thread fringe and thereby complete an electrical circuit. If, on the other hand, a gripping collet has grasped a warp thread fringe which is made of plastics material and is thus an insulator, this lies between the two contact surfaces, so that the electrical circuit is not completed and consequently no signal is issued. However, there is a problem in that dirt forms in the gripping collets and thus also in the switch, which after an unforeseeable period hampers the monitoring function, as the dirt acts as an insulator. The seam-weaving machine then continues to operate despite the absence of a warp thread fringe.
A further problem resides in the fact that when there is a break in the electrical signal line the monitoring function ceases, i.e. the electrical circuit can no longer be completed even if there is no warp thread fringe.